Am 2014-07-08 01:49, schrieb Benjamin Peterson:
On Mon, Jul 7, 2014, at 16:36, Andreas Maier wrote:
Am 2014-07-07 19:43, schrieb Ethan Furman:
On 07/07/2014 09:56 AM, Andreas Maier wrote:
Am 07.07.2014 17:55, schrieb Ethan Furman:
On 07/07/2014 04:22 AM, Andreas Maier wrote:
Where is the discrepancy between the documentation of == and its
default implementation on object documented?
There's seems to be no discrepancy (at least, you have not shown it),
The documentation states consistently that == tests the equality of
the value of an object. The default implementation
of == in both 2.x and 3.x tests the object identity. Is that not a
discrepancy?
One could say that the value of an object is the object itself. Since
different objects are different, then they are not equal.
but to answer the question about why the default equals operation is an
identity test:
- all objects should be equal to themselves (there is only one that
isn't, and it's weird)
I agree. But that is not a reason to conclude that different objects
(as per their identity) should be unequal. Which is
what the default implementation does.
Python cannot know which values are important in an equality test, and
which are not. So it refuses to guess.
Well, one could argue that using the address of an object for its value
equality test is pretty close to guessing, considering that given a
sensible definition of value equality, objects of different identity can
very well be equal but will always be considered unequal based on the
address.
Probably the best argument for the behavior is that "x is y" should
imply "x == y", which preludes raising an exception. No such invariant
is desired for ordering, so default implementations of < and > are not
provided in Python 3.
I agree that "x is y" should imply "x == y".
The problem of the default implementation is that "x is not y" implies
"x != y" and that may or may not be true under a sensible definition of
equality.
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